HYPOXIA-INDUCED PRB HYPOPHOSPHORYLATION RESULTS FROM DOWN-REGULATION OF CDK AND UP-REGULATION OF PP1 ACTIVITIES

Exposure of CV-1P cells to hypoxic conditions results in reversible ce ll cycle arrest concomitant with accumulation of pRB in the hypophosph orylated, growth suppressive form. Similar to cell cycle arrest induce d by serum starvation, we show here that hypoxia-induced arrest is acc ompanied by a decrease in pRB-directed CDK4 and CDK2 activities, lower cyclin D and E protein levels, and by an increase in p27 protein abun dance. Immunoprecipitation studies reveal an increase in p27 associati on with cyclin E-CDK2 complexes. In contrast to cell cycle arrest indu ced by serum starvation, hypoxia increases PP1-mediated pRB dephosphor ylation. These data reveal that synergy between decreased pRB-directed cyclin/CDK activity and increased pRB-directed phosphatase activity c ontribute towards inducing and maintaining pRB in its hypophosphorylat ed, growth suppressive state during hypoxia.